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mmm m conMiinioN

STORAGE EESERyOIRS,

AETHUE JACOB, B. A.,

NEW YOEK:

D. VAN NOSTRAND, PUBLISHER,

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THE DESIGMNG ASD CONSTKCCTIOif

STOEAGE RESERVOIRS.

Before entering upoa such con side ration a
as affect the selection of reservoir sites and
their construction, a brief allusion, to some
of the most ancient worka for impounding
water may not be uninteresting. Of these
the most prominent examples are undoubt-
edly to be found in Hindostan, where the
magnitude and antiquity of the storage
works cannot fail to arrest attention. These
great works, surpassing in their immensity
what are conventionally esteemed to be the
wonders of the world, the production of
other countries and nations, took their ori-
gin in the neceasitiea of the people and the
a of the climate of India, and

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were, in fact, great public ■ ivoris on which
the welfare of the people mainly depended.
The climate of India, although singularly
uniform in some respects from year to year,
is remarkably variable as regards the rain-
fall; and in order to guard against the dis-
asters of famine and eiekaess, inevitably
attendant on a scanty monsoon, the native
princes were wont to make such provisions
as large resources and an almost unlimited
power enabled them, in order to obviate
the difficulty that they had to contend with.
The rain records of India for several
years past show that a scarcity of rain ia
indicated by periods of about five years, or
that every fifth or sixth year is marked by
a scanty rainfall over certain districts. The
recurrence of these periods is, of course,
not very clearly marked, but still it is sufii-
ciently so to warrant, with approximate
correctEess, the prediction of scarcity and
famine ; and such deplorable recurrence is,
as all are aware, now reigning in India,
and visiting with destruction, by i
and hunger, some thousands whose si
pendence is upon a fair season of raii

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tile Bucceasful maturing of their little crop
of grain.

The natural expedient for guarding
against the recurrence of these periodical
calamities was evidently to be found in
husbandiDg a scanty supply of rain-water
for the purpose of irrigation, and this the
people of India appear to have understood.
They took advantage, in certain districta, of
every nook and ravine, whether large or
email, and converted them into storage re-
servoirs by throwing across banks of earth,
or bunds, as they are t«rmed, producing, in
certain districts, such an elaborate and
complete system of irrigation as can only be
compared, for cost and completenesj, to our
railway system in England. Taking four-
teen districts in the Madras Presidency,
Tvhore tank irrigation was most generally
relied upon, the records of the Indian Gov-
ernment show that there are no less than
43,000 irrigation reservoiw now in effective
operation, and as many as 10,000 more that
have fallen into disuse, making a total
number of 53,000 storage works. The ave-
rage length of embankment is found to be

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about half a mile, the extreme limit of the
series being a dam of the immenae length
of 30 miles. This ancient reservoir, called
the Poniary tank, is no longer in use, the
cost of maintaining such a length of bank
in adequate repair having probably been
found disproportionate to the advantage
derived from the supply. The work, em-
bracing an area of storage of between 60 and
SO Bq. miles, remains however as a record
of what the Hindoos are capable of. To
quote a second example, there is the Vera-
num reservoir, now in actual operation as
a source of supply, and yielding a net rev-
enue of no less than £11,450 per annum.
The area of the tank is 35 sq. miles, and
the storage is effected by a dam of 12 miles
in length. In order to bring the immensi-
ty of this system of storage works within
the reach of statistical minds, it has been
calculated that the embankments contain
as much earth as would servo to encircle
the globe with a belt of 6 ft. in thickness.
To show that these are not singular exam-
ples, one other embankment of remarkable
size may be alluded to. This embankment,

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of eomewhat singular oonBtruetion, was
built on the ielanii of Ceylon, and bears
testimony that the' Singalese nionarchs
■were not behind their neighbors in public
spirit or enterprise. The embankment was
composed of huge blocks of atone strongly
cemented together, and covered o^er with
turf, a solid barrier of 15 miles m length,
100 ft. iride at base, sloping to a top nidth
of 40 ft,, and extending across the lower
end of a spacious valley

Thus it will appeir that the practice of
embanking across valleys, for the purpose
of retaining the surface water, has for ages
been in operation There is no doubt that
the disposal of some of the most remarka-
ble works in India is not what it might, with
advantage, have been ; the fact remains,
however, that the desired end was attained,
and if the earthworks were dispropor-
tionately estensiye, it was a source of satis-
faction at least for the projectors to know
that they cost, aa a general rule, little or
nothing, the practice in those days being to
press ■whatever labor ■was required, render-
ing in return nominal wages or none at
aU.

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The two main questions that it la pro-
posed to submit for consideration are, first,
the selection of a reservoir site ; and, se-
condly, the leading principles to he observ-
ed in the designing and construction of
storage works.

The purpose or purposes for which the
work may be required will, of course, af-
fect materially the choice of a position, as
well as the details of the structure itself;
but certain general principles are available
for our guidance in every case, after con-
sideriog which, it ia proposed to dwell upon
Euch points as apply to the special purposes
for which reservoirs may be constructed.

The first and most essential point for ae-
curato determination by the engineer ia
undoubtedly the amount of rainfall, both
maximum and minimum, that may be ex-
pected in the district under examination ;
and, having arrived at reliable data on this
point, the next consideration will obviously
be, what amounl may be made available,
due allowance having been made fur evap-
oration and absorption. When we know
that the annual depth of rainfall taken all

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over the world varies, according to the lo-
cality, betweea zero and 338 in. or 28 ft.
rieep {which excessive amount was on one
occasion registered in the hill district of
"Western India), it will be obvious how
little ground there will be for assumption,
in the examination of any district hitherto
unexplored, with regard to the question of
its rainfall. In the examination of any
given country, however, there are certain
phenomena connected with the rainfall that
will be found of almost invariable accepta-
tion, and may with advantage be borne in

The rainfall will, as a general rale, be
greatest in those districts that are situated
towards the point from which the prevail-
ing winds blow. If Great Britain for in-
stance be taken, the western districts will
be found the most rainy. The very reverse,
however, of this phenomenon is noticed in
the neighborhood of mountain ranges. If
the wind prevails from one side rather than
from the other, it is found that the greatest
rainfall is on the leeward side of the range,
and the probable solution of the mfitter is,

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that the air, highly charged with moisture,
is carried up the hills by the wind until it
comes into a cold region of the atmosphere.
Condensation of the watery vapor immedi-
ately takes place, and the result is a fail of
rain on the side of the mountain range re-
mote from the prevailing wind.

To this cause may also be attributed the
fact that the rainfall is always greatest in
mountainous districts, while it by no means
follows that elevated plains are more abun-
dantly supplied with rain than land lying
nearer to the sea level. The principles are
remarkably exemplified in the southern
part of the Bombay Presidency, where the
author has had occasion to study the sub-
ject of rainfall. The Western Ghauts run
parallel to the coast, rising to a height of
4,500 ft. above the sea, and form the west-
ern support of the great table-land of the
Deccan, the mean elevation of which may
be taken at 3,000 ft. In the rainy season
the south-west monsoon, blowing from the
sea, impinges against the ghauts, and while
passing onwards to the Deecan, parts with
its moisture to the average annual amount

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of 254 in. On a spur of mountain that
runa eastward, the pluviometers are found
to register but 50 in. ; and about 40 miles
farther inland the rainfall is not more in
some places than 15 in., irhich is consider-
ably less than that registered in the lower-
lying districts of the Presidency.

In civilized countries like our own much
valuable information is as usually avail-
able regarding ths rainfall, if not ap-
plying actually to the district under exam-
ination, then probably to some neighbor-
ing districts, enjoying the same physical
characteristics; but when any project of
great importance is in contemplation, it will
not be sufficient to take the returns of ad-
joining districts as accurate information of
the rainfall at the exact locality fixed upon
for the construction of the works. It will
be necessary to establish rain-gauges at
different points over the catchment basin
of the valley from which it is intended to
obtain the supply ; and daily observations
of these gauges must be taken for compari-
son with a series of simultaneous observa-
tions taken and recorded at the nearest sta-

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at which the rainfall has been regu-
larly and carefully noted. It ia evident
i eompariaon of tlie several obserra-
taken over the area of water-shed with
registered at the pormaneDt station,
will convey a juSt estimate of the amount of
maximum and minimum rainfall that may
be relied upon.

The amount of rain falling upon the
ground is not, however, the point to be
determined, though it will aid considerably
as a guide to the engineer. A considerable
quantity of all the rainfall is either absorb-
ed by the ground or evaporated before it
reaches the point at which it can be made
available for storage. Regarding, then, first
the question of absorption, it must be ap-
parent that no two districts, unless they are
exactly identical in soil, inclination of sur-
face, and under similar circumstances of
cultivation, can give on examination the
same comparative result of rainfall and
evaporation. If one district or unit of area
be siniilar to the other in all respects but
the surface inclination, that which has the
greatest slope will, as a rule, give the larg-

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e of wafer available for stoc-
lUTse there will be less
time for the rain to be absorbed. Again,
the degree of cultivation will materially af-
fect the result when two areas, otherwise
precisely similar in their physical conforma-
tion, come fo be compared one with the
other, it being evident tbat an open and
well-drained soil will be more favorable to
the retention of water falling upon it than
compact and impervious land. In every
case the physical features of a district will
each and every one of them, force itself on
the attention, as influencing tbe conclusion
to be arrived at. If any general rule can
be applied, it may be said that the greater
the slope of the valley, the more rapidly
it will throw surface water off ; the more
denuded the surfiice is of soil of any kind,
the less will the escape of rain-water be re-
tarded ; and the more compact the rocks
composing the geological strneture of a dis-
trict, the better will the circumstances be
for impounding water. The volcanic rocks
and those of the granite order will be as fa-
vorable as any that could be desired;

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while, on the other hand, poroua rocts,
Buch aa the sandstones, chalk, etc., are too
absorbent to offer the desired conditions for
storage. It is not here asserted that all
the irater absorbed by porous rocks is ne-
cessarily intercepted from passing away to
contribute to storage supply ; much of it
may be lost by evaporation and absorption
by vegetables, but a considerable portion
■will often be found to contribute in the
form of springs, if the disposition of the
strata be favorable.

As a further source of loss, evaporation
from the ground as well as from the sur-
face of the reservoir, must be taken into
consideration. The circumstances attending
the latter source of loss will be considered
further on, as this does not affect the ques-
tion of bow much of the total rainfall may
be made available.

The question how much wafer will be
evaporated at any moment from the surface
of land is one involved in considerable diffi-
culty ; and so many disturbing elements
enter into the solution of the problem, (bat
its accurate determination may be'regarded

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as hardly possible of attaiament. The hy-
grometric state of the grouad's sur&ce, the
aspect of the sky, the amouat of wind, and
the temperature, will all, in their degree,
exercise a sensible influence on the amount
of water that the ground will give off from
its surface ; so that, in fact, it is doubtful
whether any reliable and philosophically
correct eoaeluaions can be arrived at. The
resultant facts from such experiments as
have heea carefully conducted afford, after
all, the only data for the engineer to arrive
at any general conclusion by; and for form-
ing a rough estimate for the probable avail-
able rainfall of a district, the following
proportions of available actual rainfall may
be accepted as fumisbiag geaeral data ; but
they are not meant to obviate the necessity
of a careful and specific examination of the
circumstances lijiely to affect the design of
any particular work :

Steep sur&ces nf granite, gneis^ and fJate 100

Moorland and hill pi-ature fiO to 80

Flat cultivated country 4I> to 50

ChaU Oto

In order to arrive at more specific, aad
truly reliable results, the engineer will have

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to make a series of accurate observations
on the discharge uf the stream or streams
that carry away the rainfall of a district ;
and by doing so, and at the same time
comparing the result with the amount of
rain, registered by the gauges — which
should also, of course, be kept with accura-
cy in the locality under examination — an
approximately true estimate of tlie availa-
able rainfall will be arrived at.

If there is time in the preparation of a
project to make the necessary examination
of a district, it is evident that the results
■will speak for themselves ; and there will
be no necessity to enter into abstract specu-
lations concerning the theory of the influ-
ences affecting loss by either evaporation or

In proportioning the ze f a t
reservoir to the area of th t hm t b
ill, the engineer will, of th fi

instance be guided by th q t

the work. The object f tl u d t k
may be any one of the f 1! w g

To husband a scanty r f 11

To check the injuriou H t p tt
country by floods.

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To add to the discharge of a stream, by
preventing tbe escape of the flood waters.

The amount of storage will always be
part of an engineer's data in designing
works. It will either be his object to store
the whole of the water tbat the drainage
area will afford, which will be the ease in
impounding water for irrigation, for exam-
ple; or a certain fixed demand, governed
by the want of a town or other require-
ments, will determine tbe amount of the
rainfall' that it will be necessary to retain
for supply. In England the demand for
water supply may be reckoned at from 150
to 180 days, depending on the amount and
the constancy of the rainfall ; as a rule, the
sis months' supply will be the eafest to
adopt. The following Table, extracted
from Mr. Beardmore's work, showa the pro-
portions that have been observed in design-
ing some of tbe best constructed reservoirs.
{See Table A.)

From this it appears that the proportion
between the amount stored and the total
rainfall varies between one-half and one-
fottith.

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LOCAUTT.

icht
eSea,

1
1

Greenock, 18J7-2S, flat moor

ft, ft.

40OtoS80l)
612 m 11(01)
200 to 350
7H to 1600
850 to 1600

fiq. m.
5.15

(ilencoree, Pentklid'Hilia ..

6.00

Kivington Pike, m7

LoniieiidalB "

Tunon and EntwUlUi'isSS

800 to 1545
SOOtolBOO
bOOtolSUO

16.25

"■3 lis"

B'jllon Waterworks

Ashion " 1841.,,.

800
800

,0 1600

!69

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Table A — Cif/itinued.

■s

„

•S

I-

lai

.= i

«l

£t;

■-I

■a
o

■1

■|

1=)

c f

e ft

c f n

iwr

pe m

mil

K* 6

2!) '

141b 6
834

^1
3 9

63 U

S«

6(0 y

100 (

i 3

S 10

61 i
41^8

lib 4

50
6 S

63 i
b<

ao 8

4J1 3

:46 i

49 8

2880

16 7

40
3

S
413

29 6

e 7

46 2

31 43

64S 2

i" S

lO" i

125 i

3-2

2 6

] 1

40.,

40.0

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The rule suggested by Professor Ean-
kine " for estimating the available capacity
required in a store reeerToir, that founded
upon taking into account the supply as well
as the demand," is probably the best that
can be adopted in designing waterworks for
the supply of a town; "for example, 180
days of the excess of the daily demand
above the least daily supply, as aseertaiaed
by gauging and computation in the manner
above described." In order that a reser-
voir of the capacity "prescribed by the
preceding rule may be efficient, it is essen-
tia! that the least available annual rainfall
of the gathering grounds should be suffi-
cient to supply a year's demand for water.''
In calculating the capacity of a storage re-
servoir, the consideration of the surface
evaporation must not be disregarded, espe-
cially when the works are designed for
tropical or very dry climates. The amount
of loss will in some cases be very consider-
able, for whatever depth of water be assum-
ed to pass away into the air, it must be re-
garded as extending over the whole surlacs
of the reservoir ; or, in fact, the cubic quan-

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tily will be eqiial to tbe product of the
depth evaporftted away and iho mean sur-
face area of the reservoir as the water rises
or falls throughout the year. Some have
gone the length of asserting that the amount
of evaporation from the surface of lar^e and
deep bodies of water is probably nothing at
all, or, at any rate, not worthy of considera-
tion ; whilst others assume a much larger
amount of loss than appears to be support-
ed by observation. The following extract
from the article "Physical Geography,"
published by the Society for Promoting
Useful Knowledge, expresses intelligibly
the cooditions that tend to promote evapora-
tion:

"Other things being equal, evaporation
is the more abundant the greater the
warmth of the air above that of the eva-
porating body, and least of all when their
temperature is the same. Neither does
much take place whenever tJie atmosphere
is more than 15 deg. colder than the sur-
face upon which it acts. Winds powerfully
promote evaporation, because they bring the

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air into continual as ■well aa into closer and
more violent contact witk the surface acted
upon, and also, in the case of liquids, in-
crease by the agitation which they occasion,
the number of points of contact between the
atmosphere and the liquid.

" In the temperatfl zone, with a mean
temperature of 52^ deg., the annual evap-
oration has been found to be between 3fi
in. and 37 in. At Cumana, oa the coast of
South America (N. lat. 10^), with a mean
temperature of 81.86 deg., it was ascertain-
ed to be more than 100 in. in the coarse of
the year; at Guadaloupe, in the "West Indies,
it has been observed to amount to 97 in.
The degree of evaporation very much de-
pends upoa the difference between the
quantity of vapor which the surrounding
air is able to contain when saturated and
the quantity which it actually contains. M.
Humboldt found that in the torrid zone the
quantity of vapor contained in the air is
much nearer to the point of saturation than
in the temperate zone. The evaporation
within the tropics, and in hot weather in
les, is on this account less

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than might have been supposed from the
increase of temperature."

Thus jt appears that evaporation, under
highly favorable conditions, may take place
to the extent of 9 ft in depth — an allow-
ance that will demand careful conaideratioa
in designing storage works. In India,
where from the extreme dryness of the at-
mosphere the evaporation is found to be
considerable, the usual allowance made by
engineers for the evaporation from the sur-
face of storage reservoirs is at the rate of
J in. of depth per diem for eight months in
the year. Eegarding the results that have
been arrived at in Bombay, this allowance
would appear to be about double what is
necessary, for the observations eztending
over five years give a mean daily evapora-
tion of less than J in. In Bombay, how-
ever, the atmosphere is much more humid
than that experienced on the great table-
land of the Deecan ; and in Madras, where
reservoirs are the specialty, it is probable
that the actual loss is not far from being a
mean between the two fractions. In Great
Britain the mean daily evaporation is found
to average less than the tenth of an inch.

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In estimating the quantity of storage
water that will result from t!ie drainage of
any particular district, it will be essential
to consider carefully the geological disposi-
tion of the strata characterizing the locality
in which it is contemplated to establish the
worka. This, although a matter tliat may
inSueuce the effectiveness of an undertaking
to the extent of success or failure, will ap-
pear to the purely practical man to imply a
degree of refinement that is uncalled for.
There will be no difficulty, however, in
showing that the geological conformation
of a district may be such as, ou the one
hand, to materially contribute to the effi-
ciency of a storage reservoir, or on the oth-
er to prove so defective that no engineering
skill or pecuniary outlay could remedy it.
A condition of geological structure perhaps
the most favorable that could be imagined
is that shown in Fig. 1. This diagram
represents a geological section taken at
right angles, or nearly so, to the axis of the
valley that it is proposed to convert to the
purpose of storage. This somewhat peculiar
structure is what is geologically termed syn-

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f| i

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clinal, tile beda inclining away from the
axia of tlie valley, and ia the result of an
upheaving force having taken place under-
neath the pointa of greatest elevation. Sub-
sequent to the upheaval and consequent
displacement of the strata, the process of
denudation has taken place, cutting the up-
per beds, and leaving the outcrop e
not only inaide the baain, but in the
ing valleys at and 0. Now, It is
that if the highest ridgea bounding the val-
ley be taken to mark the line of water-
shed, and therefore limiting the area of tha
catchment baain, it ia poasible that the esti-
mate of the amount of supply may be found
far short of what the district will yield, A
certain proportion of the rain falliog upon
the outcrop at the points O will be ab-
sorbed by such of the strata as are porous,
and the water, percolating through the bed-
ding, till an impervious stratum is met
with, will find its way down the course of
the atratifieation, till it ultimately reaches
the reservoir in the form of springs, and
contributea more or less to the maintenance
of the supply. The converao of thia oondi-

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tjon of things will be readily understood by
reference to Fig. 2. It also represents a
section taken directly across the valley of
the proposed reservoir. Here the strata of
the earth's crust incline against each other
consequent upon some disturbing force
having taken place to elevate them, dnd
are said to be anticlinal to the axis of the
valley. In order to account for the forma-
tion of a valley on the summit of the ridge,
that at first was thrown up, it is to be un-
derstood that the upper beds suffered frac-
ture in the process of upheaval, and subse-
quently were exposed to denudation. These
valleys of elevation are evidently not to be
desired as situations for the establishment
of storage reservoirs. The area of the
gathering grounds will be much more lim-
ited than the extent of the watershed would
appear to indicate ; and cannot safely be
relied upon to give an estimateof the quan-
tity of w t that th alley will afford. A
certain a o nt f water will undoubtedly
pass ov th fa n times of heavy and
continu i an b f o it can be absorbed ;
but the n d ubt that of all the water

b, Google

absorbed by the ground, \iy far the greater
portion will follow tlie inclination of the
strata, and come out as springs in the ad-
joining valleys.

Fig. 3 shows a geological section that
combines in it favorable and unfavorable
conditions for the storage of water. On one
side the outerops of tho strata are found to
extend beyond the highest point of water-
shed line, whilst on the other side the strata
incline away, producing such a condition
as would favor the escape from the valley
of the water absorbed.

Certain rules are in general use for esti-
mating the quantity of the total rainfall
that will be lost by absorption and evapo-
ration, with a view to determining the
proper proportion to be observed between
the reservoir and the area of the catchment
basin. Two-thirds of tho whole fall is
sometimes taken to represent the loss that
may be expected from the drainage of any
district, in general terms, one-third being
assumed as the amount that may actual-
ly be intercepted for utilization. ?ome au-
thors leave a much smaller margin, and

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Btate that fully two thirds of the total rain-
fall may fairly be taken as available for
storage. This ia a large discrepancy when
the application of the rules ia taken to be
general ; but when the statements are ap-
plied to separate districts and different
countries, there is nothing irreconcilable in
them. General rules are undoubtedly of
much value if they be received with quali-
fication, and are not adopted as of absolute;-
ly universal application. They cannot,
however, with safety be substituted for
specific investigations, when so much de-
pends on starting with accurate data.

The special requirements of each partic-
ular case will, as a general rule, go far
towards determining the selection of a site
for the establishment of storage works.
Assuming, however, that there ia a consid-
erable extent of country situated advanta-
geously in relative position f« the locality
at which it is proposed to utilize the water,
and that there is a choice of ground, the
point to be considered chiefly will be the

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naf ral lie of the country. To tlirow an
embankmeflt across a valley at aoy point
■without due regard to tlie configuration of
the ground would most probably result in
an expensive and ill-designed scheme ;
for under such ciroiirastancea the cost of tho
dam would bear a very large proportion to
the quantify of water stored. It will rarely
happen that, in the examination of the re-
sources of any particular piece of country,
some special features will not present them-
selves, favorable to the situation of storage
works. The most advantageous disposition
of the ground will be when two spurs of
high land approach each other, forming a
narrow outlet for the stroam, and leaving a
wide space above above them ia the valley
for storage. Such a configuration is not
uncommonly met with at the junction of
two streams, as shown in Fig 4. This is
merely a sketch from memory, by the
author, of a reservoir that lie designed in
India for purposes of irrigation ; and it will
be evident that the disposition of the
ground was singularly favorable in every
respect for the construction of a large

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storage work. The area of the i
as designed, was about three square miles,
and the maximum depth 90 ft., the area of
the catchment basin being about 60 square
miles- Such favorable situations for storage
are of somewhat rate oeeurreneo ; for when
the contour of the land is what is desirable,
it may be that the area of water-shed is
not adequate, or possibly the geological
condition of the ground may be unfavorable,
or the materials for the construction of a
sound bank are not available. In examin-
ing large tracts of country iu India, with a
view to the establishment of irrigation re-
servoirs, the author found that more reli-
ance ■was to be placed on' a careful exami-
nation of the map in the first instaace,
than on the common plan of making per-
sonal espbratious of the country. A good
map will show at a glance, especially if the
hill-shading has been carefully engraved,
the points at which the supply will be found
sufficient to justify the undertaking ; and
will probably furnish a pretty true indica-
tion of sites at which embankments may be
advantagpously constructed.

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In tropical climates, where the rainfall is
in places very scanty, and wiiere the land ia
not of great value, it not unfrequently hap-
pens that ench situations prove available
for the establishment of large storage
works as would not under any circumstan-
ces be made available in England. These
sites are to be found, not at the head of a
valley, but at some considerable distance
down the course of a stream, where, the
general inclination of the country being
slight, a low embankment serves to store a
very large area of water. The apparent
disadvantages of such a site for storage are
the large area of land swamped and lost to
the cultivator and to Government, and the
great surface exposed to evaporation under
a tropical sun and the influence of a dry
wind. In India, the first objection is on©
of comparatively little moment, considering
that in those districts where irrigation ia
most required the value of land is very
trifling. From la. to 2b. is about an average
rent per acre, where land is under dry
crops; but when .Tratar is available, the
cultivators can, with profit, afford to pay

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30s. per acre. It is therefore evident that,
Eo far as Government is concerned, there is
no sacrifice in the matter, but, on the con-
trary, an unspeakable benefit is conferred
on those landowners who hold farms below
the reservoir ; and an ample supply of water
is stored in the dryest seasons to mature
those crops whose failure almost inoTitably
reduces the people to the verge of starva-
tion. The evaporation from these lakes is,
beyond question, a source of very consider-
able loss, and one that admits of no possible
abatement. Estimated as above, at about
half an inch vertical for eight months of the
y&T, the loss frequently amounts to one-
third of the whole body of water stored.
As a set-off against this and otlier objec-
tions, the facilities for constructing these
reservoirs of great extent, are considerable.
In the first place, the embankments, being
very low, are rapidly and cheaply construct-
ed by native workmen ; and when finished,
the head of water even at the deepest point
is not sufficient to try the work to any great
extent. Further, the greater the extent of
the reservoir, the less inconvenience is eS-

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perienced from silting. The streams, owing
to the suddenneas of the rainfall, comedown
heavily charged with earth in suspension,
the mass of which is deposited like a minia-
ture delta at the influx of the reservoir, in-
stead of passing on and resting near the
embankment, as invariably occurs in reser-
voirs of small estent The immense coji-
aumptioQ of water necessary to confer any
appreciable benefit by irrigation is of itself
the Btrongest argument in favor of these
broad and shallow reservoirs ; for it is not
possible to find in the upper part of a. valley
such sites as would store the requisite quan-
tity of water without an embankment of
excessive dimensions ; and moreover, the
catchment .area in such situations is not
usually sufficient to serve, with a scanty
rainfall, for the supply of a very large re-
servoir. It is not, of course, maintained
that this mode of storing water is by any
means applicable in England, for the cir-
cumstances and requirements in each casa
are wholly dissimilar.

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The reservoir site being supposed every-
thing that could be desired, as regards the
disposition of the ground, the supply will
next engage attention as a matter of courBe.
Assniniiig that the gathering grounds
are sufficiently extensive, it is presumed
that the reservoir will be constructed to
contain sufficient water to meet the maxi-
mum demand, whatever that may he calcu-
lated at ; and in order to determine with
accuracy what capacity the reservoir will
have with different heights of embankment,
it will be necessary to carry out certain level-
ling operations over the ground. The least
elaborate manner of proceeding will be to
riiQ a series of cross-levels through the
valley, referring all to the same datum, and
by comparing these levels to ascertain what
the average depth will be for a given height
of bank. Having decided the height of the
water-level, the next operation will be to
contour round the basin, and to survey the
boundary-line. In this way may he acquir-
ed sufficient knowledge as lo the storage
capacity, to justify the procedure with the

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work. When the execution of the project
hB8 been determined upon, it will be advis-
able to make a more accurate suryey of the
bed of the valley, and this can best be done
by covering the whole plan with a series of
contour lines at a vertical distance from
each other of about 5 ft. This bind, of
survey will be of lasting value to the engi-
neer, for it will enable him to calculate
what quantity of water the reservoir will
contain at each foot of depth ; and, conse-
quently, lie will know, from a mere inspec-
tion of the gauge in the reservoir, how
much water he has at his disposal for
service.

It has been assumed that the gathering
grounds are sufficient to maintain the re-
quisite supply in the reservoir ; but it may
be well to pause and inquire what estentof
water-shed will be sufficient to furnish a
given supply, and what method may be
adopted for supplementing an insufficient
drainage area. It has before been remark-
ed that the only reliable information, when
there is any question as to the sufficiency
of the rainfall or the area of the catchment

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basin, can be dei-ived froDi carefu.] gaugings
of tbe stream or streams that may be depend-
ed upon to contribute to tbe supply. If the
catchment area is very large as compared to
the ca[ a ty of the re ervo r a me e n pec
t on of tho map and an eiplo at on of the
gro nrt \i 11 generally be conclu ve as to
the euffic ency of the 6 pplv for storage
Sh uld there n t be such c n 1 ve ev
lence on th 3 p nt t must be dete mined
by meis g the quant ty of water that
absolutely flo vs off the ground a the s me
time gauging the rainfall. This latter pve-
caution would appear unnecessary, but in
truth it ia of great value, for it will furnish,
by comparison with the rainfall registers
that have been kept through the same year,
and a series of previous years, evidence as
to the amount of available rainfall that may
be expected during terms of comparative
drought. If the supply of a town with
water be the desideratum, the rule to be
rigidly observed ia that of making a mini-
mum supply meet the maximum demand,
and therefore it is of the highest impor-
tance to determine beyond any doubt, what

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the minimum yield of a catchment ba^in
wiU be

Asamfdeof supflementmg an msufEi
cieutly large di am age area eatohment drains
or fee lirs have hequently rendered good
service T leae are cuts tliat are carried
uutside the wafer shed hue to anest the
curface diamaf,e aud catch the contribu
tioua of small etrearas iud eynduct the
water luto the re^iervuir TJie gre iter the
area enclosed between tatchnient diams
and the water shed hue the more valuable
i\dl they be as aids to the supply of the
rtservoir Thev of course virtually e\teiid
the area of the catchment, adding -^o m-my
square milesor acres to the rainfall.

BESieuiHG or woEKS.
Knowing the exact requirement of a
given population, or rather having fixed,
after every consideration, the daily con-
sumption of every individual that it is pro-
posed to supply, there will be no difficulty
whatever in proportioning the reservoir to
the demand upon it. It is sometimes ne-
cessary, however, to provide reservoirs for

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t!ie purpose of preventing damage to the
country by flooJa, and in this way the in-
conyenience and injury naturally conse-
quent upon very sudden and excessive falls
of rain may be to a great extent obviated.
The duty of the reservoir will be to arrest
aU water in excess of what the stream can
carry within its banks, and to dispose of this
excess water, so to speak, in detail, after the
excessive rainfall has become moderated,
A comparison of a stream's discharge, taken
at highest floods, with the quantity that it
can carry without overflowing its banks will
show the excess that has to be retained by
the reservoir ; and these data can only be
arrived at throuj;h a carefully keft record
of tl e extent of the floods and of their du
ration The max mum floiJ m this con
eider ation will not be that ■which uses to
the giedtest he ght for a short time but
will be the product of the exi-eas ab Vf>
what the ruer can disci i (,e bv the length
of time the flood lasts i) ch. will in ta t
be the na essdry capa t3 of the re^er

The T iblp gi\ en on a J I "-ceding \ ige will

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afford an inteio&tmg btudv whHn uiuparpil
with the following Table i>stiattpd in piit
from the same woit The fitst gives a
comparative viuw of the vclume of water
ganged ind <itore 1 in tmall hill diitncts
the list column iniicafm,}; the proportion of
the total available rainfall to the amount
actually intercepted Jor stoiige The fol
lowing Table shows the ordinary "ummer
discharge of v^njus ruers, streimi and
sprmgB, as unaffcoted by immediite rain
{See Table B )

Where the reserioir is designed to check
the mjunoui effects of fl lods the pi por
tion of the storige to the ramfall will in
most cases be much smaller than what
would be necessarj to piovide for the
better part of a whole year s fill jf rain
for lb is ujt probable that the maximum
known flood can ever exceed tla amount
that it would be necessary to -itoro for
economic purposes.

PKOPOKTIONS Of EAXK.

The proper proportion to bo given to an
embankment for the support of water is a

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RIV1..RS.

fll«ve He;i.

Thames at 8taine>, cluilk, Rreonsand,
Oxford o\ay, o..liles,6lc

LcddoQ (Fehniarj, 18501, sreeE,«Bnd,
NeiiP, at Pelerhorough, oulites.

Valley Hill.
ft.' ft.

to Co 700
400 to S60O
110 to 700

Idimraiu. at Fsnsliantrer. chnlk....

Lee, at Lee Bridge, chalk (Eennie
April, 1796)... . . ..... ..

WnEidle, below CanhaltDD, chalk. . .

200 to 500

80 to 600
70 to 350

Ditto, ordimry summer nin(Reniue,

VernUm, at Bushej Hall, chalk. . . .
Gsde, at Hunton Br'id^. chalk. . . . .

Hym, at Sheepstor, Krunite

Wov-dhead Tunnel, millBtoiie, grit..

JSOto 5m
150 to 600
800 to 1500

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TATii.E B — Continued.

1
1

30S6
3900
221.8

620.0
60.0

570
il.O

4Sl.e

na'.s

695
7.6

per min,

40,000
33.111
3,000

5,000
1,200

8,880
1,800

2,209

2,S20
1800

'500
139
130

e. ft,
p«r mm.

8!49
13.63

8.45
2.4

4 59

5 23
1+.9

n'.i

in.

3 63
9 !I3

3.19
3 37
8 19
15.10

23.1

'2i;o

6.0

21 6

4 9

37.4

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question that appears to admit of a good
deal of difference of opinion, some designers
taking one view, some another, of the pro-
per theory tliat is to determine the diatea-
siona of a bank. Some few, with whom
the author cannot agree on this point,
maintain that a bank ought to be designed
with strict reference to its theoretical power
of resisting hydrostatic pressure, or the
effort of the water to displace it Regarding
the question in its abtfract foim, it will be
evident that any structuie intended to sus-
tain the prpssure of water may be supposed
to tail in one of two !( ays— either, m the
first place by jielding to the hoiizontal
pleasure of the water and overturning, or
by piogiebsive motion, t <■ sliding on its
base In considering the first theuiy, that
of resistance to oveitiirmng, the easiest
method ot eiimmmgthe question will be
to take J simple example of a vertical
rectangular will, and ascertain what poner
it ei.eicises to resist the pressure ot v ■iter
The prcssuie of water upon «iy plane
Burfice immersed is known to be equal to
the area ot tliat surface, multiplied hy the

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depth of ita centre of gravity below the
level of the water aud by the weight of a
unit of water. Generally speaking, the
unit adopted in calculations is a foot ; and
the unit of water being taken at a cubic
foot, weighing 62.5 Iba., the resulting pro-
duct from the multiplication of the three
quantities will give the pressure in pounds
on the surface immersed. Let it be sup-

posed, for simplioity, that water to the
depth of 10 ft. has to be suBlained by a
vertical rectangular wall, as in Fig, 5. It
is usual to take but 1 ft. length of the

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wall for the calculation, though it will not
affect the result whether 1 ft or 100 ft
be the length asaumeil "We then have
the surface under pressure =:: lU aq ft , the
depth of the centre of giavity^5 ft, and
the weight of a cubic foot = b2 5 lbs , tha
product of which <juantitie=! gives us 3,125
lbs. pressure on 1 tt length of the will But
this pressure is not the whole of the force
that the wall taa to resist , the leverage
that it exerts must also be taken into
account. In the esample under cou'iider
ation — viz. that of a vertu al plane with one
of its sides coinciding with the suiface of
the water, as in 1 ig 5 — the whole ot the
pressure is so distributed is to be equal to
a single force acting at a point one third of
tlie depth from the bottom Ihus, the
total force to be resisted by the wall is
2125 X 3.83=: 10,40G, which IS the moment
tending to overturn the wall

It is evident that a certain weight of the
wall must be oppo&ed to this overturning
force ; and as the height of the wall and
the length are determined quantities, the
thictness alone remains for adjustment

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But as a rectangular ■wall ia upsetting is
considered *o turn upon a single point, F,
in the Figu'.'e — viz. the outer line of ftie
■wall — Uiere will be a certain amount of
leverage, to assist the wall in resisting
the pressure of the -water. This leverage
is the horinontal distance of the centre of
gravity of the wall from the turning point,
P, and when the structure is rectangular
and vertical, it is equal to half the thick-
ness. The amount of the ■wall's resistance
will then be equal to the number of cubic
feet in one foot of its length multiplied by
the weight of a single cube foot of masonry
and by half the thickness of the -wall.
Taking w^ weight of a cubic foot of water
^62.5 lbs., w*=w6igbt of a cubic foot
of brickwork, say 112 lbs., iC = tbicknes3
of the wall, and A=the height, the con-
ditions of simple stability will be fulfilled

M'XftX»iX| = «'XAx|x| (1)

and solving for x, we g

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the thiclitiesa of the wall ^ 4 ft. 4 in.

A simple example has been selected for
illuatration but of course a reetangular
section of wall would not be found gene-
rally applicable in practice, nor would it be
cjnvement to limit the dimensions of a
retaining wall of whatever kind to the
mmimum that would sustain the pressure.
It this principle of calculation be applied to
ascertain the stability of a bank of earth
■Rith long slopes of 2^ or 3 to 1, it can
easily be shown that in every case the
resistance of the bank to overturning is
greatly m excess of the horizontal leverage
exercised by the water sustained.

The only theory, then, in any degree
tenable, is that assuming a bank in yield-
ing to the pressure of water to slide on its
base. In order to conceive how this can
apply, it is necessary to assume the em-
bankment to be a rigid body resting, for a
given length of its section, on a horizontal
plane ; and without any adhesion, or a very
small fraction, existing between the sur-

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faces pressed. The amount of the friction,
however, is just the point upon which the-
whole matter hinges, and nntil it has been
ascertained that the surfaces of earth that
are carefully incorporated with one another
have any snch thing as a co-efficient of
friction, it is idle to to puisne the investiga
tion by a mathematical mode of reasoning
The conditions of stability mil be satisfied
■when the horizontal component of the
water's pressure agamst the bank will
equal the weight of the banlv, plus the
vertical pressure exere ted bj the naterto-
hold it down and multi[ilied by the co-
efficient of friction , but nothing la known
of this CO effirient, and conseijuently the
equation remains incapable of solution
Aa a matter of fact, enibauliments do not
slide bodily foiwird on then base when
they fail, but gi\e way from other causes
than mathematical reasoning can supply
Landslips, it is true, to tome extent support
the pnncijle that maintains the shding of
embankments but, here, the circumstances
are widely different Ijandslips either take
place when a mass of earth rests upon an

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inclined surface of rock, with an ample
supply of water to lubricate the surfaces in
contact, or else they are the result of cutting
or embanking earth to a higher slope than
the material wUI stand at ; the iafiitration
of water also in this case is the chief agent
in producing the effect, acting as a lubricant,
and causing the earth to assume its natural
slope. In each case the surface of separa-
tion is an inclined plane, an element that
does not enter into the question of the
stability of embankments, by either of the
modes of reasoning above referred to. The
principles that direct the design of embank-
ments to retain water are not those that
apply to the calculation of the forces to be
resisted or the means to overcome them,
any more than breakwaters and harbor
walls can be designed on mathematical
principles. The whole question naturally
turns on what elope the material composing
the bank will stand at. If earth could be
got to remain at a slope of 1 to 1, even
though the embankment had no thickness
■whatever at top, it would be amply sufScient
in weight to uphold the water in a reser-

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Toir. This, lioweyer, cannot be aecomplished.
without the assistance of retaiuiug trails,
whioh would be found in most eases much,
more expensive than the additional earth
required to increase the slope to the angle
of stability; and therefore the section is-
BO disposed that the earth shall stand both
inside and outside the reservoir at such a
slope as will be under ail circumstances
permanent. These slopes have been deter-
mined by long practice and by success and
failure in pre-existing instances — that ia to
say, the limits have been laid down, for it
is not to bo assumed that all descriptions of
earth will fall to exactly the same slope
when exposed to the constant action of water
or weather. Earth when subjected to the
contact of water almost invariably loses a
certain amount of its stability, and there-
fore it is usual to give the inner aide of an
embankment a longer slope than the out-
side. In most of the best existing examples
the inside slope of the bank is either 3 to 1
or 2^ to 1, and it is rare to meet any de-
parture from this rule. The outside slope
may be designed at from 2 to 1 to 3 to 1,

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depending upon the character of the material,
its power of withstanding the erosive action
of the air, and the means used to protect
the snrface from heing washed off or from
crumbling away. In designing embank-
ments, the impermeability of the earth is a
matter that cannot be relied upon. There
are, it is true, tnnnmerable embankments
now standing that have never allowed the
escape of a drop of water from the
reservoir, although no special precaution
was taken to make them water-tight. Of
these India abounds with examples, the
introduction of a paddle wall being in the
older embankments of very exceptional
occurrence. Tbe earth was merely dug out
close at hand, and carried by the work-
people in baskets on their heads to where
it was depositod, without any regard to the
mode of disposing the material. The
author has had occasion to construct a
considerable length of levee, or embank-
ment, on this simple plan for the protection
of the country from the flooding of a river ;
and although, so far as he is aware, no
flood has yet taken place to teat the work.

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he has, from the study of existing examples,
entire confideECO in the result. The earth,
so far aa practicable, was disposed in layers,
and before each was completed it was
thoroughly consolidateii by the tread of
the workmen. It is not suggested that the
puddle wall should be dispensed with in
designing embankments, for the additional
degree of safety, in most instances, will
more than compensate for the extra expense
it entails; but, in low embankments made
of good retentive clay, the precaution of
puddling is by no means a necessity.

In most of the best examples of embank-
ments in England, the practice adopted has
been to carry up the earthwork iu layers of
2 or 3 ft. in thickness, disposed in the man-
ner shown in Tigs. 6 and 7, and at the
same time to construct ia the centre of the
bank a wall of well-puddled clay, the foun-
dation of which is carried down for what-
ever depth may be necessary in order to
reach an impermeable bed of earth or rock.
It is not in all situations possible to pro-
cure earth exactly suitable and in sufficient
quantity for the construction of an embank-

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^ \

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ment, and consequently it is usual and ad-
visable to dispose the best part of the ma-
terial — that is, the most retentive of water
— in juxtaposition to the puddle wall, as
indicated in 'Fig. 6. In this exaoiple, the
selected material is disposed equally at ei-
ther side of the puddle ; but, as its func-
tion is to withstand the admission of water,
it would probably be more consistent, though
less in accordance with practice, to place all
the selected material on the inner side.
The practice of excavating the earth for an
embankment from the inside of the reser-
voir is one that should not be followed
without caution. Eemoving so lai^e a
mass of material would, no doubt, give a
considerable increase of storage room ; but
sometimes the bed of a reservoir is covered
by a layer of impervious clay that is of im-
mense value, and if this be cut through
or removed, it is quite possible that a bed
of porous material may he met with suffi-
cient to allow the escape of water when it
comes to be admitted. In specifying for
the dimensions of the puddle wall, a sound
rule for adoption is, that it shall have a

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thickness of 10 ft. at the top water-line and
increase in thickness to the surface of the
ground at the late of 1 in. on each side for
every foot of height. Before any excava-
tion is commencod, it will be essential to
make a sufficiont number of borings to as-
certain the nature of the soil beneath the

It may here be mentioned that profes-
sional men are not apparently agreed as to
the principles to be kept in view in con-
structing reservoir embankments ; and thia
■want of concurrence never waa more appa-
rent than in the discussion that followed
the destruction of the Dale Dyke reaervoir,
near Sheffield. Fig. 8 shows a plan of the
embankment site after the catastrophe.
The bank was 95 ft. high, with slopes of 2|
to 1, and a top width of 12 ft. The pnddle
wall was 1 6 ft. in width at the ground-line,
and tapered to 4 ft. at the top of the bank.
This embankment, with the exception of
the puddle wall, was composed of rubble
stone and shale ; an additional price hav-
ing been given by the engineers to insure
the use of the former materia! ; which

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p t J t that this mode of Goa-

ti t w 1 J t d oa priaeiple aad not
th gh r mistake. From the

d g by the eagiaeers of the oom-

p J t [ J th t it was, in their opin-

d bl th t the iaaer part of the
mb km t h ild be permeable to water,
b tl w much aiore likely to aub-

d d 1 p th opea aad less jieldiag

m t Ilk t Tiiia mode of construo-

t mpl th t tl e puddle shall be fully
ffi t f t If t resbt the passage of
t d th t th e is ao aecessity to re-

1 t f y p t f the pressure agaiaat

t Of f baak be composed of

p k y po Dt in. the face of the

p d 11 po d t the full and direct hy-

d f t I nd if at aay poiat there

th m II t fi 'e or iniperfectioa, the
w t h f 11 p w agaiast it, and will, to
ta t k J. aatiige of such poiat to

breach the dam. The assumptioa, then, of
the coas'ructors of this and the Agdea res-
ervoir evidently was that a puddle wall of
some 25,000 sq. ft. of area was to be con-
structed without an imperfection of any kind,

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or a single weak point in tlie whole sur-
face.

The obvious reason for employing pud-
dle at all, in embankments, is to thorough-
ly close up any imperfection, that may occur
in the earthwork ; it is in fact merely an
accessory, and cannot be relied upon of it-
self to secure the embankment against de-
atruetion. If an embankment be construct-
ed of good Bound earthwork, properly exe-
cut«d, it is highly probable that the water
may never penetrate half way through to
the puddle wall, and probablj, in the ma-
jority of examples, has not done so. Earth-
work, however, is not always executed
without imperfection; some decomposable
material may be introduced, which, in
course of time, dissolves, leaving a fissure ;
one part may be at first less consolidated
than another, and, subsiding, lead to imper-
fection ; or an embankment, be it ever so
well eonatructed, may be burrowed through
by moles, rats, and other vermin. It is to
meet the first two of these sources of imper-
fection that puddle is used ; and if, by such
i as may occur in ordinary earth-

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work, water is admitted as far a9 the pud-
dle wall, it can only oxercise pressuie
against it at a few points, the puddle and
earth being, in good work, so bonded and
incorporated with each other that there is
no space left for the ■« ater to occupy and
press against the surface. Most who have
read the account of the disaster that occur-
red in March, 1864, at Sheffield, will recol-
lect how aiugularly conflicting the profes-
sional evidence on that occasion was. Some
of our first engineers were ranged against
each other in order to satisfy the public aa
to whether the failure of the embankment
was attributable to bad engineering or to a
landslip ; and although the impression
finally remained on the public mind that
" there was not that engineering skill and
attention to the construction of the works
that their magnitude and importance de-
manded," the engineers were fairly divided
in opinion aa to the cause of the disaster.
One section pronounced, without qualifica-
tion, that the embankment gave way in
consequence of a landslip, aad entirely
i the fact of t

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defectively constructed ; whist the other
geutlemea gave their verdict dead against
the company, and their mode of construct-
ing water-tight banks. The two diagrams,
Nos. 6 and 7, may be taken as indicating
the system of constructing embankments
most generally approved of. The puddle,
as will be observed, is carried up to tha
natural surface of the ground without any
batter, and from that point slopes on each
aide to the lop of the bank ; on either sid»
of the puddle is disposed, in concave layers,
the most sound aud retentive part of the
material, and outside of all comes the ordi-
nary earthwork.

As a security against the eroding action
of the water, and also against the inroads
of vermin, the most desirable, as well as the
most usual practice, is to pitch the whole of
the inner faoe of an embankment with
atone, carefully laid by hand. Neglect of
this precaution has led to the destruction of
many embaukmenta in other respects se-
curely constructed, and even when ample
height of bank above the surface of highest
water was provided. In all ordinarily in-

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clement weather tbe disturbance of the
surface of a reservoir amounts to no more
than a mere ripple ; but when the surface
is of large extent, and a severe storm blow-
ing, the ivavi's produced are such as to
cause reasonable apprehension, and, in fact,
have, before now, overtopped the bank and
cut it down, till the water flowed over and
■caused the destruction of the work. lu
most eases, it will be necessary to leave
about 5 ft. between the level of the highest
water and the top of the embankment, and
never less than '6 ft.

A. mode of construction not very gener-
ally used, but apparently consistent with
reason, is that shown in Pig. 7, the em-
bankment for the Eideford Waterworks.
It consists in covering the whole of the
f w th 1 J f I ddl w th some-
tm ly

til
1

a t

1 t d

.ome
ble to

I ddl

, tt

f smaU
y f ob-

h tiBbj

p t, th utl m d d f asonry

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dam in India that had to be pointed every
year regularly, because the fteah-water
crabs in the reservoir found it couvenieut
and promotive of their development of shell
to appropriate the mortar to their personal
use. The joints were cleaned out as effec-
tually at the end of each monsoon as if the
work had been done to order.

The preparation of the foundation for an
embankment is a matter requiring some
care. The soil, consisting of grass, roots,
etc., and other matters of a decomposable
mature, should be carefully rom.oved over
the whole surface to be covered by the
bank, and if any porous material, such as
saud or gravel, be present, it must be re-
moved, until a compact and water-tight bed
is arrived at. The bank must, in fact, be
in contact with some sound and reliable
material that will not admit the passe^e of
water.

APPENDAGES OF EESEfiVOIUS.

Under this heading may be considered:
The whole apparatus for allowing the

water to escape, including the pipns, the

valve tower, and the culvert.

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The waste sJuices.

The waste weir or by- wash.

The most economical oiodo of disehai^ing
water from a reservoir is through a single
jiipe passing either through the embank-
ment or immediately under it ; but this
plan cannot, under any circumslances, be
recommended, though it is.some times found
in existing examples. It is open to several
grave objections, the principal of which,
perhaps, ia that the failure of a joint under
the embankment from unequal pressure, or
fi-om whatever cause, will probably produce
the destruction of the embankment, or ^t
any rate, entail a serious interruption to the
supply, by the reservoir having to be
emptied in order to repair the pipe. Buried
in or under an embankment, a pipe is com-
pletely out of reach and out of view, and
may be in a very defective state without its
being possible to detect the imperfection.

In order to secure the satisfactory work-
ing of a reservoir as a source of constant
supply, it is essential that the outlet pipes,
Talves, and all other appendages for con-
troUiDg and regulating the escape of the

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water, should be accessible for inspection
and repair. The iisual mode of accom-
plishing this is to carry the pipes out
through a culvert of brick or masonry of
sufficient dimensions to admit a man. This
culvert communicates with the valve tower,
as shown in Figs. 6 and 7, so that there is
a complete communication between tlie out-
side of the reservoir and the inside. When
unavoidable, the culvert is carried straight
under the embankment in the natuval
ground ; but the safest and most generally
approved mode of construction is to bring
the culvert round the end of the embank-
ment^ where it will be out of reach of in-
jury from unequal settlement ; a source of
no small apprehension when either culvert
or pipes alone are carried under the banli,
Wheie posBible, it is an excellent plan to
run a heading through the solid ground,
lining it with brickwork and puddling it,
forming a tunnel entirely independent of
the embankment. The principal objection
to carrying either the culvert ot pipes
through or under the bank is their liability
to fracture from the unequal settlement of

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the earthwork. It would appear that their
liability to damage cannot with certainty be
insured by any reasonable depth of excava-
tion, and ia, therefore, generally disapproved
of by the beat authorities.

In the beat construotions the culvert is
Bitaated half way or two-thirda up the em-
bankment, and in auch ease the outlet pipea
for drawing off the water in the reservoir
aot as syphona when the water surface has
fallen below the culvert. Pig. 6 shows a
plan, as well as a cross section, of a reser-
voir dam designed for general application
by Mr. Eawlinaon. Here the bottom of the
culvert is about 25 ft. above where the in-
ner slope of the embankment interaects the
ground at the loweat point. The syphon
pipe is alao ehowu paasing through the
culvert ; the horizontal culvert is connected
■with a shaft inside the embankment, in
which are placed the valves for leading off
the supply from the reaervoir. The valves
are made to be closed on the inside by
valve spindles and acrews, and the inlet
pipea are closed on the outside by plugs
which can be applied from the top of the

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yalve-tower. Thus the engineer has full
command of the whole of the outlet works ;
all the pipes and valves are easily accessible
and under perfect control, so that the supply
can at any time be arrested for the repair
of any derangement that may occur, even
to the removal and replacement of all the
pipes. The inlet pipes are shown in this
example, as well as in Fig. 7, fised at
different heights in the valve-tower, the
object of which is to draw the supply from
the reservoir from points near the surface.
The outlet pipe, passing through or under
the embantment, may be connected on the
inside of the reservoir by a flexible joint
■with another pipe of the same diameter, to
the upper end of which is attached a float.
This pipe is movable in a vertical plane,
being controlled from lateral motion by the
guide-posts. Such an arrangement admits
of the water being drawn off from the sur-
face, where it is least liable to be contami-
nated with impurities. Whatever arrange-
ment be selected for drawing the supply
off froni a reservoir, the system of carrying
the pipes, either with or without a culvert,

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through or under the embankment, cannot
be sufficiently deprecated ; they are, in
such a position, beyond the reach of iaspec-
tion, and, moreover, are very likely to induce
leakage from the reservoir. It is usual to
puddle carefully the culvert or pipes vrhen
carried under or through the bank, but, even
with such a. precaution, the water has under
a considerable head a tendency to creep
along the pipe, and, by soaking into the
earthwork, may cause any one of the many
evils that imperil and destroy embank-
ments.

When embankments are not of great
height, an exceedingly cheap and simple
mode might be adopted for drawing off the
water. This would be by laying a syphon
over the embankment, as was done in the
case of the middle-level drainage in Cam-
bridgeshire, which syphon would at the
inner side have a flexible connection with
another tube having a float attached, as
above described. Such an arrangement
would apply in principle to heights not ex-
ceeding 30 ft., as the pressure of the atmos-
phere would maintain no greater height, In

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practice, however, tlie syphons cannot be
vrorked with success at mucli above 20 ft.,
for it is found that after a short time, the
flow becomes arrested by the ooUsction of
air in the upper part of the syphon, and it
becomes necessary to pump the air outeon-
Btantly, to prevent it from interfering with
the flow, aa it would do if not removed. It
would appear a simple matter, where it is
desirable to adopt a syphon, to utilize the
power of the water flowing out for the pur-
pose of getting rid of the air; it might
easily be applied, through a small wheel
and suitable gearing, to work an air-pump
fixed at the highest point of the syphon,
making the whole arrangement self-acting.
The arrangement could be successfully ap-
plied to irrigation tanks in India, where the
embankments are frequently less than 30
ft. Each leg of the syphon should be pro-
vided with a valve to retain the water, and.
when the supply was intermittent it would,
be essential to have an opening at the
highest point of the syphon, and some ap-
pliance, perhaps an air-pump, for filling ife
with water in case of leakage.

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sure a constant discharge from a
■ with, a constantly varying head,
several methods have been adopted; of these,
one of the most ingenious is that used at
the Gorbals "Waterworks, near Glasgow.
Fig, 9 represents a transverse section
through the regulafor-house, showing the
arrangement by which the dischaige is
equalized. To the orifice of the outlet pipe, 0,
is fitted a square-hinged flap valve of wood,
against which presses, by a friction roller,
a levor, B, the arms of which are bent. To
the upper arm is attached a chain that pass-
es over a pulley, and is connected with a
cait-iron cylinder or float, D, that stands in
the reservoir, B, of slightly larger diameter.
At the side of the entrance-door of the
building is placed another cistern, G, of
cast-iron, closed at top, and communicating
by a pipe, II E, with the vertical pipe, H,
which is in connection with the outlet pipe,
and passes up the slope of the embank,
ment, to carry away any air that may ac-
cumulate in the main. The cistern, G, ia
connected with the reservoir, E, by a pipe,
K, which supplies water to float the cyl-

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inder, I). Now, it is evident that fhe dis-
charge from the reservoir will bo regulated
by the position of the lever, B, and thia
again will be controlled by the height of
the float, D. To regulate this height the
supply from the cistern, G, must be self-
adjusting, or be regulated by the ataount
of water flowing away. The float, N, has
attached to it a spindle, on which are fixed
two double-beat valves that work in the
vertical part of the pipe, K, one of which
admits water from the cistern, G, into the
cylinder, E, and the other allows the water
to escape from the reservoir, E. Now, if
the surface of the water upon which the
float, N, rests should rise above the proper
level, the float forces up the spindle, closing
the supply valve from the cistern, and at
the same time opening the lower valve.
Thus the supply is cut off and the escape
opened, enabling the float, I), to fail. The
Bubsidence of the float closes more or less
the flap valve, and checks the dischai^e, in
consequence of which tho surface of the
■water falls, and with it the float, N, which
consequently opens the supply valve, and

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again admits water into the cistern, E.
Thus an almost perfect equality between
the consumption and the supply of water is
preserved. It would appear that the same
effect could be produced by connecting the
lever directly with a float on the surface of
the water, but such an arrangement would
only apply when the pressure against the
flap is trifling.

It is essential that every reservoir should
be provided with some means of gettingrid
of the excess of water that flows into it, and
whether this provision be made by a waste
weir, sluices, or waste pit, it is one that
should not be omitted. The most advan-
tageous position for a waste woir will
generally be at some point remote from,
and entirely unconnected with, the em-
bankment, and occasionally a natural de-
pression in the ground, as shown in Fig- 4,
will afford remarkable facilities for the
construction o£ an escape. The level of the
crest of the waste weir with reference to
the top of the dam will require to be care-
fully adjusted, the minimum difference of
level being 3 ft., and the niasimum about

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1[) ft., depending on varying circumstances.
The height of the waste weir will, of course,
regulate the top water level in the reservoir ;
and this miist be fixed with regard to the
probability of the embaniment being over-
topped by waves. The circumstances in-
fluencing the height of the waves in a
reservoir are the extent of the water surface,
the depth, and ihe amount of espoaure to
or shelter from wind, all of which will vary
"with each particuaJar case. Under ordinary
circumstances, the height of the top of the
embankment above the creat of the waste
weir should be for

an embankment 25 ft. deep, 4 ft,

50 ft. " 5 ft.

75 ft. " 6 ft.
and for greater height of embankment the
difierence of level may be proportionately

When the configuration of the ground
does not afford any facilities for the con-
struction of a waste weir aft«r the manner
described, sufficient provision for the escape
of the overflow is made through a waste
pit. This waste pit, or lower, is generally

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a circular structure built over the outlet
culvert inside the reservoir, and serves
equally for access to the valves and for the
escape of the flood water. With regard to
the capacity of the waste weir or waste pit,
whichever be adopted, it will be necessary
to make ample provision for the discharge
of the sudden accessions of flood water that
reservoirs are subjeot to, and which so
seriously imperil their safety. To provide
for this there is an empirical rule amongst
engineers that is supposed to suffice for the
most urgent contingencies. It states that
there shall not be less than 3 ft. of length
of overfall for every hundred acres of
gathering ground, but it is obvious that to
proportion the length of the waste weir to a
given area of country in all cases would bo
unreasonable.

The discharge over the weir will not
depend only upon the quantity of rain fall-
ing on a certain area of ground, but also on
the extent of the reservoir as compared to
the gathering ground, and on the flat or
precipitous character of the basin. The only
eafe mode, then, of proportioning the length

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of the escape will be to ascertain witli
esactaesB what the discharge of the stream
or streama flowing out of the reservoir was
during the greatest known flood, and then
fixing upon an arbitrary depth for the
water to flow over the weir, say 2 ft. or 3
ft., to calculate what length of overfall will
suffice for the discharge of the excess
water. In India, where large waste-weir
accomodation is essentially necessary, while
it is equally a necessity to save every
gallon of water that ia possible, it ia a
common practice to form a temporary dam,
of earth and sods, on the top of the waste
weir; this serves to pond up some 3 ft, or
4 ft. of water over the whole snrface of the
reservoir, and does not imperil the security
of the works. In times of heavy floods the
water rises and overtops the temporary dam,
and no sooner does so, than the whole ia
carried away, and the water in the reservoir
quickly subsides.

In works designed for the supply of
towua, it is aometimes necessary to make
provision to arrest the entrance of flood-
water into the reservoir, as the streams may

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come down charged with large quantities of
roatter in auspensioa that would injure the
|mrity of the water for domestic consump-
tion. These streams may be diverted and
carried round the margin of the tank past
the dam, and can be admitted into the
channel of the stream, or be utilized for
mill power. On the Manchester Water-
works are constructed across the mountain
streams weirs of an ingenious design, for
the purpose of separating the flood-watera
irom the ordinary flow. The dimensions
are adjusted from observations of each par-
ticular stream, so that the dischai^e up to
a certain amount will take place into the
channel for the supply of the town ; but
when the discharge increases, and the water
becomes turbid, it has sufficient velocity to
carry it over the opening, as shown in the
diagram, and flows down to the compen-
sation reservoir for the supply of mill
power.

In determining the dimensions of a weir
of this kind it is first to be ascertained
what the mean velocity of the water flow-
ing over will be for a given depth of water.

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A, above the crest. The mean velocity, u,
will be

« = | X H 034 Va'^ 5.3.5 v'ST

If tbe vertical height of the crest of tha
weir above the point to be overleaped by
tiie cascade be called x, the distauce across
will be

Via ^

Before coucluding, it will be well to give
a brief consideration to ihe causea tending
to the failure of embankments. The fore-
going remarks will, in suggesting the best
mode of conatracf ion, have anticipated much
that might be said on the subject of failures ;
but there are a few points, the recapitula-
tion of which the importance of the subject
demands.

There are unfortunately on record, acci-
dents, if they can be so called, from the
bursting of embankments, that if estimated
by the loss of life attending them, are as
appalling aa anything within the memory of
man. Thousands of human lives have been
BscrLficed to ignorance and false economy,

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as well as in' some instances to natural de-
fects that it would have been difficult to
foresee.

The existence of springs on the site of an
embankment is an undoubted cause for ap-
prehension, and considerable care should be
taken to carry all water from this source
away, that it may not, as it certainly will if
not checked, force its way between ihe sur-
face of the ground and the seat of the em-
bankment- In doing so there is every prob-
ability that the earth of the embankment
will he washed out by constant trickling till
a fissure is formed of sufficient dimensions
to render the destruction of the bank a cer-
tainty, if the water from the reservoir
should ever penetrate so far. As a provis-
ion against this source of injury, all springs
found on the site of an embankment should
be taken up and carried away in proper
drains sufficiently and securely puddled.
Thus the water is confined to a single chan-
nel, and has no tendency to soak into the
earthwork and blow it up in endeavoring to
escape. In embankments of all kinds the
presence of water is a most serious evil, and

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one by which may be accounted for, some of
the most extensive land slips that are on
record. It is erroneous to assume that
when water is the active element in pro-
ducing disruption in an embankment or
mass of earth of any tind, that it only acta
as a lubricant between the surfaces in con-
tact. The truth is, the bulk of earth ia sen-
sibly affected by the amount of moisture in
it, aa is seen in the subsidence of newly-
formed railway banks when exposed to rain.
If, then, a sufficient quantity of water find its
way into the centre of a bank that has been
put together in a comparatively dry state, it
will rise and soak inla the earth until at
length what waa a solid mass becomes
semi-fluid, aettles into a smaller space thaa
it before occupied, and, as a consequence,
will leave a vacuity above it. The inevita-
ble result ia the subsidence of the superin-
cumbent earth ; but instead of resting, as
at first, on a resisting material, it floats, so
to speak, on the semi-fluid mass underneath,
and having little or no friction to overcome,
slips away to a lower angle than it before
stood at. Natural springs, therefore, when-

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ever they occur, must be dealt with care-
fully and completely. Exactly similar
effects to those produced by natural springs
may result from the defective practice of
carrying outlet pipes through or immedi-
ately under embankments. Be the pipes
ever so well puddled, there will be a ten-
dency to trickling along the line of their
direction, and assuredly if this trickle makes
its way to the centre of the bank it will
carry mischief with it. It is true that
springs are occasionally found issuing from
the foot of an embankment, without after
several years causing any appearances
to justify apprehension, The Doe-park
reservoir is an example in point, and though
at one time fears for its safety were enter-
tained, the embankment is still standing,
and, so far aa the author is aware, the
spring is still trickling away. An engineer
of eminence waa called upon to report upon
the stat« of the works, and gave his opinion
that, as the spring came away without any
earth in suspension, there was no mischief
taking place, and that the work was in a
safe condition. There ia no doubt that em-

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bankmeata ia this condition require to be
narrowly watched, although the presump-
tion may be that, having lasted for several
years, they will continue in safety.

The empirical and unscientific mode of
proportioning the length of waste weirs has
proved before now a source of danger and
destruction to embankments, from the space
afforded not being sufficient to discharge
the excess water without the surface rising
to such a height as to top the embankment.
To avoid risk, the stream must be gauged
with great care, and the discharge calculat-
ed for the greatest known flood ; and if
with a given head the length of the weir be
adjusted to discharge this amount, or a
little in excess, there will be no risk to the
embankment.

Regarding finally the whole subject, the
danger that may result from careless or
unscientific construction, the large outlay
entailed in the establishment of storage
works, and the benefit that may accrue
from them whatever their purpose may be,
the subject cannot be undertaken on merely
rational grounds. Its successful applica-

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tion will rest alone on the study of the
question in its scientific details, and an
ample practical experience.

BrHCCSsio:^.

Mr. n. P. Stephenson said he entirely
agreed with the author as to the impro-
priety of can-ying a pipe through the em-
bankment of a reservoir. He would extend
his objection to the passing of a culvert
through the embankment. If the culvert
were laid on the natural ground, they
would avoid the risks pointed out by the
author, either of the settlement from the
joints of the pipe, or of the water creeping
along between the material and the pipe.
He believed that the true principle of con-
struction for reservoirs was the placing of
a good puddle dam in the centre, and on
each side of this dam layers of earth well
punned in. One reason why be should
prefer the puddle wall m the centre was
that there was less tendency in the puddle
to slip in such a position than when laid on
the slope.

Mr. Albert I>atham agreed with Mr.

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Stephenson in his remarks as to the pipes
and culverts ; but he thought it was an
open question whether the puddle wall
should be in the centre of the dam. He
had a strong opinion that it should be on
the face of the dam.

Mr. Cavgill said that he believed that the
reason the puddle wall was not required in
Indian embankments, referred to by the
author of the paper, was that the earth
seemed to have been thoroughly consolidat-
ed by the continual trample of people upon
it. That thorough consolidation was the
great point in all puddling, and it was on
that account that specifications were gener-
ally BO stringent as to the thickness of the
layers of the puddle. As to the position of
the puddle wall, he could not see the parti-
cular value of having it in the middle of the
dam, and he thought that a far better place
for it would be the face, because the object
of the puddle wall was to prevent the infil-
tration or the escape of the water. This
could be effected by puddling the whole
elope tight down to the permanent strata.
The puddle wall was not required to

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promote the stability of the dam. Tlie
question of putting pipes or culverts under
the dam required more consideration.
It was alleged that the putting of a naked
pipe through the dam of the Bradfield re-
servoir was one of the causes of its bursting.
In BOme very lai^e waterworks now being
constructed in Dublin there were two dis-
tinct sets of main pipes, and they were laid
in two large culverta at the bottom of the
dam. The culverts were large enough for
a man to walk upright iu them. If the
foundation were well looked after, there
would be no fear of the arch or dome of the
culvert giving way ia consequence of any
inequality of pressure above it, as, if pro-
perly constructed, an arch would stand any
amount of pressure short of what would
crush the material.

Mr. Baldwin Latham said he could not
agree with Mr. Jacob that a dam could not
be constructed from theoretical deductions ;
for unless regard was paid to theoretical
couBiderationa there might result either a
deficiency of strength or a waste of material
and labor. In the dam shown in the draw-

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ings, and designed by himself, the pipe did
not run through, but on the outside of the
dam, on the aohd ground. It was a well
received opinion among engineers that if
you had a pipe or culvert running through
an embankment, that pipe or culvert would
be unsafe. He believed that well made
and properly tested pipes were quite as safe
aa culverts when in the solid ground. A pipe
was simply a small culvert made of iron
instead of brickwork. In cases ia which
there was a tendency for the wafer to creep
' along the outside of the pipe, that might be
stopped by having projecting flanges on the
pipe. The same creeping of water might
take place along a culvert as along a pipe.
With regard to the slope of a dam, the in-
side slope should be greater than the out-
side slope, because the greater would be
the stability of the dam, and the water
would Lave less destructive effect on the
dam ; he had effectually prevented leak^e
by the use of socket-pipes. The square
projection of the sockets was alway present-
ed to the reservoir, and the pipes were laid
in the virgin ground. It was very bad

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practice to lay tlie pipes in made ground,
and especially tlirougli a dam. Pipes laid
under a dam should be tested under pressure
after being laid and before being covered
up, so tliat any defective joint might be
discovered. In cases in which he had laid
pipes through dams, they had heen so
tested, which resulted in good and effective
■work ; but he was bound to say that, if the
pipes had not been tested in situ the result
■would not have heen satisfactory.

Mr. Schonheyder said that Mr. Jacob had
said that wherever springs occurred they
should be well carried away. He (Mr.
Schonheyder) wished lo know how a spring
was to be provontod from diffusing through
the earth.

Mr. Hendry said that he had seen pipes
which were laid through embankments,
but had never seen one that was perfectly
tight. It was almost impracticable to make
it so, owing to the continuity of the puddle
being disturbed at the point where the pipe
passes through.

The Chairman asked what was the
largest diameter of pipe Mr. Hendry had
eeen used.

b, Google

Mr. Hendry replied that the largest was
18 in. He had heard of several methods
being tried, but he did not think it waa
possible to prevent leaking, more or less,
from the reservoir along the outside of the
pipe. He ehould like to be informed how
it was possible to connect the puddle with
the pij e if the i ii es be lai i in the nafura'
grounl below the foundation of the em
bankment then there is no fear of leak
age, provided the l-iies are properly
laid

Mr Jacoh m replying to the discussion
eaid that m the cj-mions that had been
espre 'ed there were but fnw points of
disagreement with those that he himself
held. He could not agree with Mr. Latham
in his belief that embankments could be
calculated on mathematical principles. In
order to deal with embankments tlieoret-
ically, they must be regarded as rigid
masses, and be assumed to rest upon a
horizontal plane. It could be shown math-
ematically that a rigid body of the same
speciBc gravity as ordinary earth need not
present the same section as is usually given

b, Google

to embankments, in order adequately to
resist th.e pressure of water. Aright angle
prism with the hypothenuse resting upon
the plane wouldbe quite sufficient to resist the
pressure of water, even supposing the sur-
face of the water to coincide with tlie upper
edge of the prism. The reason of giving
long slopes to an embankment is discover-
able from the fact that banks, when exposed
to the action of water, are found to waste and
slip away to such an angle as will withstand
the action of the water. The chief reason of
the failure of embankments is the infiltration
or soaking of the water from the inner side,
which renders the material semi-fluid and
causes it to subside into a smaller space than
it originally occupied. The superincumbent
mass then sinks and allows the water to
overtop the embankment. The earth used
for making embankments in the Deccan
and in parts of the Madras Presidency in
India is of a most suitable quality for the
purpose. It is what is called " black soil,"
being very dark in color, and of a highly
argillaceous charaoter. The color is, no
■doubt, due to the presence of carbon. The

b, Google

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